1. Field of the Invention
The present invention relates to a head-up display (hereinbelow, referred to as HUD) and a combiner used for the HUD.
2. Discussion of Background
Recently, there has been used a HUD in order to display information to a driver in a vehicle. The HUD is so constructed that the driver can read information without substantially moving his eyepoint at the drivers seat by displaying an optical information projected from an information projecting means such as a liquid crystal display device or the like on a combiner such as a hologram, a half mirror or the like which is installed in or on a windshield glass or the like of the vehicle.
In particular, it is effective to use the hologram as the combiner of the HUD and to diffract an optical information to the eyesight direction of the driver. It is because the hologram possesses a feature capable of diffracting an optical information to the driver as well as functioning as a lens; a feature capable of forming an image at an optional position without using an optical system such as a lens or the like, and a feature capable of displaying an image having high brightness without deteriorating brightness of the foreground.
FIG. 17 is a diagram showing an example of a conventional HUD. Light 43 including information to be displayed, emitted from a light source 46 and passed through a transparent type liquid crystal display element 45 through a lens system 44, is irradiated on a hologram 42 arranged in a windshield glass 47 of a vehicle body. The light is diffracted at the hologram 42 to be seen by the driver at his observing position 41.
The lens system 44 functions as a collimator. Further, the lens system 44 may be omitted by sharing the function to the hologram 42. By sharing the function of a lens to the hologram 42, a speed displaying image 48 and an alarm displaying image 49 can be formed at a distant place. Since the conventional hologram 42 has a specified wavelength selective function because the half width of the diffraction spectrum is narrow as about 5-20 nm, it is possible to display an image having a color light of desired wavelength. Usually, the color to be displayed is single. However, a display having plural colors is possible, so that the quantity and quality of a displayed information can be improved. For instance, a speed displaying image 48 is shown in a green color and an alarm display image 49 is red whereby it is possible to transmit information to the driver correctly.
As described above, the half width of the diffraction spectrum of the hologram used for the conventional HUD is as narrow as about 5-20 nm. Accordingly, the hologram diffracts light having a specified wavelength. When the driver looks at something outside through the hologram, the color of light transmitted through the hologram (hereinbelow, referred to as transmission color) is the complimentary color of a displayed color corresponding to the diffraction wavelength of the hologram. Namely, as shown in FIG. 11, when a displayed color is only green, a white light 22 from outside is partly reflected and diffracted by a diffraction grating in the hologram during the transmitting of the light through the hologram 21, and the reflection light 23 becomes green. Accordingly, the transmission color (the color of transmitting light 24) is magenta (pink-red) as the complimentary color of the diffraction light and the driver may feel stimulative and uncomfortable to the transmission color. Further, the color tone of the background is also influenced by the complimentary color, and the visibility to circumstances such as a road outside the vehicle is impaired during cruising whereby a problem in driving safety may be caused.
On the other hand, as shown in FIG. 12, with respect to an observer 35 outside of the vehicle (a walking passenger or a driver in an opposing car), the color of reflection light of an outer light 32 by the hologram 31 (hereinbelow, referred to as reflection color) is substantially changed from a red color 33 (observation from the front) to a green color 34 (observation from a slant position) depending on an angle of observation. Due to the angle dependence of a diffraction wavelength by the diffraction grating in the hologram 31. The color change also gives uncomfortable impression.
FIG. 15 is a chromaticity diagram showing the color change of reflection colors. The chromaticity diagram is to show quantitatively colors by the x and y coordinates of the chromaticity coordinates ruled in Z8701 of JIS (Japan Industry Standard), which provides specification of colors according to the CIE (Commission Internationale de I'Eclairage) 1931 standard colorimetric system and the CIE 1964 supplementary standard colorimetric system. A mark X (a light source) expresses a white color (standard light source D.sub.65). As the reflection color of the hologram is closer to this point (the mark X), the reflection color is closer to a white color, which is more preferable to an observer. The hologram used for the conventional HUD is a monochrome hologram having a narrow half width of diffraction spectrum. In such a hologram, for instance, a hologram whose diffraction spectrum has a peak wavelength of 545 nm, a half width of 15 nm and a diffraction efficiency of 60% as shown in FIG. 16, the reflection color of the hologram is changed as shown by marks in FIG. 15. A range of observation angle is .+-.70.degree.. It is understood that a red color is provided around a front position and there is a great color change from an orange color through a yellow color to a green color as the position of observation shifts to a more oblique position. In particular, when viewed from the front, a red color as a stimulative color is provided to give an uncomfortable impression to the observer.
In order to improve the color tone of the transmission color and the reflection color, there have been conventionally carried out to expose a hologram having peaks with a narrow half width in diffraction spectrum multiplexly to light, or to laminate a plurality of light-exposed holograms. For instance, in the HUD disclosed in Japanese Unexamined Patent Publication No. 291221/1992, an improvement of color tone is proposed by using a hologram exposed to light having two different wavelengths which are in a relation of complimentary color, or by using a hologram exposed to light having three different wavelengths corresponding to three primary colors. However, since these methods were to use a hologram or holograms having a narrow half width of diffraction spectrum, there was a limit to improve the color tone. Namely, although there were some improvements by the conventional methods, the color obtained was far from a white color.
The range of colors which can be discriminated by a human is called color difference lumen, and there is well known a test by David L. MacAdam (Journal of the Optical Society of America, Vol. 32, No. 5, p 247-274, 1942). Since the color difference lumen forms an elliptic shape of the xy chromaticity diagram, it is called MacAdam's ellipse. The magnitude of the MacAdams ellipse is not constant on the chromaticity diagram. Accordingly, an ellipse close to the standard light source D.sub.65 (x=0.3127, y=0.3290), is shown as an example. At a point near x=0.305 and y=0.323, .delta.x=.+-.0.0015 and .delta.y=.+-.0.002. It is understood that a very small difference of colors can be discriminated. However, the color difference lumen described above is unnecessary in practical use. A range of admissible color change can be within about .delta.x, .delta.y=.+-.0.05 in experience, more preferably, within about .+-.0.02.
In the conventional method wherein a plurality of holograms having a narrow half width diffraction spectrum are used, a display which satisfies the above-mentioned permissible range can not be obtained even by optimizing the diffraction wavelength. For instance, in a display comprising holograms exposed to two color lights: a first hologram having a wavelength of 545 nm, a half width of 15 nm and a diffraction efficiency of 60% and a second hologram having a wavelength of 430 nm, a half width of 15 nm and a diffraction efficiency of 60%, a change of the reflection color is shown in FIG. 13. In this case, .delta.x=about .+-.0.11 and .delta.y=about .+-.0.13. Further, in a case of a display comprising three holograms exposed to three different color lights, wherein a first hologram having a wavelength of 650 nm, a half width of 15 nm and a diffraction efficiency of 30%, a second hologram having a wavelength of 545 nm, a half width of 15 nm and a diffraction efficiency of 60% and a third hologram having a wavelength of 430 nm, a half width of 15 nm and a diffraction efficiency of 60%, a change of the reflection color is as shown in FIG. 14. In this case, .delta.x=about .+-.0.07 and .delta.y=about .+-.0.13. In either case, the degree of color change is remarkably improved in comparison with a case of the monochrome hologram shown in FIG. 15. However, the conventional methods do not satisfy the above-mentioned permissible range.
In the above, description has been made as to mainly the reflection color. However, when material for the hologram is not colored, the transmission color and the reflection color are in a relation of complimentary colors, and accordingly, the quantity of color change (.delta.x, .delta.y) is substantially the same as the color change of the reflection color.
On the other hand, when a half mirror without having wavelength selectivity is used for the combiner instead of the hologram, it is naturally possible to bring the reflection color and the transmission color close to white. However, the half mirror can not have a lens function. Further, an incident angle and a reflection angle of light including information to the combiner are the same, and accordingly, the arrangement of the half mirror in the vehicle is further restricted.